This invention relates to a semiconductor device and, more particularly, to a semiconductor saturable absorber device for use in mode-locked lasers for the generation of short and ultrashort optical pulses. The invention also relates to a mode-locked laser comprising a semiconductor saturable absorber device.
Lasers emitting short or ultrashort (in the picosecond or sub-picosecond range) pulses are known in the art. A well-known technique for short or ultrashort pulse generation is mode locking. Mode locking is a coherent superposition of longitudinal laser-cavity modes. It is forced by a temporal loss modulation which reduces the intracavity losses for a pulse within each cavity-roundtrip time. This results in an open net gain window, in which pulses only experience gain if they pass the modulator at a given time. The loss modulation can be formed either actively or passively. Active mode locking is achieved, for instance, using an acousto-optic modulator as an intracavity element, which is synchronized to the cavity-roundtrip time. However, ultra-short-pulse generation relies on passive mode-locking techniques, because only a passive shutter is fast enough to shape and stabilize ultrashort pulses. Passive mode locking relies on a saturable absorber mechanism, which produces decreasing loss with increasing optical intensity. When the saturable-absorber parameters are correctly adjusted for the laser system, stable and self-starting mode locking is obtained.
Passive mode locking can be achieved with semiconductor saturable absorber mirrors (SESAMs) (cf. U. Keller et al., xe2x80x9cSemiconductor saturable absorber mirrors (SESAMs) for femtosecond to nanosecond pulse generation in solid-state lasersxe2x80x9d, Journal of Selected Topics in Quantum Electronics (JSTQE), Vol. 2, No. 3, 435-453, 1996, incorporated herein by reference). A SESAM is a nonlinear mirror inserted inside the laser cavity. Its reflectivity is higher at higher light intensities due to absorption bleaching obtained by using semiconductors as the nonlinear material. A SESAM typically comprises a bottom mirror and the saturable absorber structure. Optionally, there may be a spacer layer and/or an additional antireflection or reflecting coating on the top surface.
For passively mode-locked lasers using SESAMs for mode-locking, the limitation on repetition rate is the onset of Q-switching instabilities (see C. Hxc3x6nninger et al., xe2x80x9cQ-switching stability limits of continuous-wave passive mode locking,xe2x80x9d J. Opt. Soc. Am. B. vol. 16, pp. 46-56, 1999). This has also limited the laser repetition rate to the range of several hundred megahertz typically. Using the technique of coupled cavity mode-locking (RPM), a repetition rate of 1 GHz was demonstrated (see U. Keller, xe2x80x9cDiode-pumped, high repetition rate, resonant passive mode-locked Nd:YLF laserxe2x80x9d, Proceedings on Advanced Solid-State Lasers, vol. 13, pp. 94-97, 1992). However this is a much more complicated laser due to the additional laser cavity which has to be carefully aligned with the main laser cavity.
When the conditions necessary to avoid the Q-switching instabilities in passively mode-locked lasers are examined more carefully, the following stability condition can be derived:
(Flaser/Fsat,laser).(Fabs/Fsat,abs) greater than xcex94Rxe2x80x83xe2x80x83(1)
where Flaser is the fluence in the laser material, Fsat,laser=h"ugr"/"sgr"laser is the saturation fluence of the laser material, h is Planck""s constant, "ugr" is the center laser frequency, "sgr"laser is the laser cross-section parameter (see W. Koechner, Solid-State Laser Engineering, 4th Edition, Springer-Verlag New York, 1996), Fabs is the fluence on the absorber device, Fsat,abs=h"ugr"/"sgr"abs-eff is the effective saturation fluence of the absorber, where "sgr"abs-eff is the effective cross-section parameter of the absorber device including a structure dependent factor and the intrinsic material cross section, and xcex94R is the modulation depth of the absorber device. This equation can be used to scale a laser for operation at higher repetition rates. If all else remains constant (i.e., mode size in laser material and on the absorber, average power, and pulsewidth), as the repetition rate increases, the left-hand term decreases due to decreasing pulse energy. It is possible to avoid Q-switching under this condition by arbitrarily decreasing the modulation depth xcex94R. However, below a certain modulation depth, the absorber will not have a strong enough effect to start and sustain mode-locking.
For further clarity we simplify Eq. (1) to the following:
Slaser. Sabs greater than xcex94Rxe2x80x83xe2x80x83(2)
where Slaser is the fluence ratio in the laser material, and Sabs is the fluence ratio on the absorber. This reduced notation allows us to simplify the further discussion. To achieve the maximum figure of merit, one can change the laser design to increase the fluence ratio Slaser in the laser material, or to increase the fluence ratio Sabs in the absorber. In this document, we concentrate on the latter measure.
In pulse generating lasers with high repetition rates, e.g. above 1 GHz, the pulse energy of course is lower for a given average power from the laser. Thus, as the pulse repetition rate goes up, it becomes increasingly harder to saturate the SESAM and thus to get modelocking. It would therefore be desirable to obtain an absorber device with a decreased saturation fluence for high repetition rate pulse generating lasers.
A reduced saturation fluence would make operation with a reduced fluence level on the SESAM possible. The beam spot size on the absorber medium could be chosen to be larger. This would be desirable for both pulse generating lasers with a high repetition rate and for pulse generating lasers operating at a high average power: A larger spot size on the absorber make cavity design easier and, very importantly, be an advantage concerning thermal issues. A very high fluence can result in optical damage. Damage levels of SESAM absorbers have been measured in the range of 30 mJ/cm2. By decreasing the saturation fluence of the absorber and by then increasing the spot size on the absorber, the laser can be kept well off the damage level. Next to possible thermal damages, a very high fluence (but still below the damage threshold) may cause the laser to operate with multiple pulses per round trip, i.e., a form of harmonic mode-locking. This may be desirable as a method to increase the repetition rate of the laser. However, it may result in decreased operation stability of the laser. Thus, Sabs is limited to about 10-30 for fundamental mode locking.
Minimum saturation fluence can be achieved by positioning the absorber medium at or near the peak of the standing wave in the SESAM.
Other absorber materials with higher cross sections, i.e., lower saturation fluences, could be found in theory. However, this is a very difficult material problem, the solution of which in the near of even far future is uncertain.
In the paper xe2x80x9cErbium-Ytterbium Waveguide Laser Mode-Locked with a Semiconductor Saturable Absorber Mirrorxe2x80x9d, IEEE Photonics Technology Letters, Vol. 12, No. 2, February 2000. E. R. Thoen et al. propose the use of a SESAM for mode-locking a waveguide laser with a resonant, multi-layer dielectric coating in order to increase the absorption and to lower the saturation fluence. A resonant structure as proposed in this paper, however, brings about high losses. It is therefore only useful in set-ups with very high gain such as the waveguide lasers disclosed in this paper. In addition, the resonance condition makes it delicate to fabricate.
More generally, resonant Fabry-Perot saturable absorbers are considered to be unsuitable as modulators for mode-locked lasers, because the loss and the group delay dispersion (GDD) which they introduce are quite high (cf. M. J. Lederer et al., xe2x80x9cAn Antiresonant Fabry-Perot Saturable Absorber for Passive Mode-Locking Fabricated by Metal-Organic Vapor Phase Epitaxy and Ion Implantation Design, Characterization, and Mode-Lockingxe2x80x9d, IEEE Journal of Quantum Electronics, Vol. 34, No. 11, November 1998).
A different question is addressed by D. Knopf et al. in xe2x80x9cAll-in-one dispersion-compensating saturable absorber mirror for compact femtosecond laser sourcesxe2x80x9d, Optics Letters 21, p.486 (1996). In this publication a combined absorber-dispersion compensating device is disclosed. Such a device is an absorber structure with means for implementing a negative Group Delay Dispersion (GDD, in this reference called Group Velocity Dispersion GVD).
According to this invention, a xe2x80x9clow-field enhancement resonantxe2x80x9d (LFR) semiconductor saturable absorber device design is proposed. In this design, the structure is changed with respect to the prior art such that it no longer satisfies the anti-resonant condition but a resonant condition. Consequently the field strength is substantially higher at the position of the absorber layer or absorber layers, resulting in a smaller saturation fluence and in a higher modulation depth.
However, the resonance is such that that the field within the resonant structure is lower than the field in free space or that it is only moderately enhanced compared to the free space field, e.g. by not more than a factor 10. As a consequence, the device does not have as narrow a band structure as resonant structure with a high field enhancement, e.g. a high finesse resonant structure.
In one embodiment, the absorber device is a Semiconductor Saturable Absorber Mirror (SESAM). In contrast with SESAMs according to the state of the art, the structure comprising the absorber, e.g. a spacer layer, is provided which essentially fulfills a resonance condition. A standing electromagnetic wave is present in the structure. In other words, the design is such that the field intensity reaches a local maximum in the vicinity of the device surface, i.e. at the device/air interface.
The advantages of the LFR-SESAM design according to the invention over the prior art are the following:
(a) A high field enhancement resonant SESAM would be difficult to fabricate, because extremely tight tolerances are required, and more temperature sensitive than the LFR-SESAM according to the invention. A LFR-SESAM is a good compromise between fabrication difficulties and improved saturation fluence.
(b) There may be no extra post-processing steps required because the LFR-SESAM is manufactured during the MBE or MOCVD fabrication. Thus, e.g. an entire LFR-SESAM structure may be fabricated in a MBE set-up without post processing. This also would be advantageous concerning the losses, since the post processed layers tend to be thicker and have more defects. It also reduces the total fabrication cost due to reduced number of processes and handling steps.
(c) The SESAM design according to the invention gives new design degrees of freedom. The modulation depth of an absorber of a given thickness and composition is increased compared to SESAMs with anti-resonant design. Thus, the absorber layer thickness may be reduced while still the modulation depth is still kept at a sufficiently high level for SESAM operation. Especially, very thin absorber layers below the critical thickness can be grown epitaxially without relaxation even when the layer material is highly lattice mismatched with the substrate.
(d) Reducing the saturation fluence and at the same time keeping the modulation depth at a customary level reduces the mode-locked-Q-switched threshold. This means that a given laser can be operated to higher repetition rates or with reduced output power without Q-switching.
(e) Reducing the saturation fluence makes possible to operate at reduced fluence levels, e.g. by reducing the spot size on the absorber. By this, intensity-dependent damages may be prevented in high power lasers or in high repetition rate lasers.
(f) The possibility to operate with larger beam spot sizes on the absorber gives additional cavity designs of freedom.
(g) Reducing the mode-locked-Q-switched threshold also means that the laser has a more xe2x80x9cgentlexe2x80x9d turn-on, i.e., it is less likely to emit a large Q-switch pulse during turn-on, which could damage the SESAM and/or other optical components in the laser or in following optical systems (e.g., components for nonlinear conversion or amplification).
(h) With the present invention, it may be even possible to design lasers that go straight from cw to cw-mode-locked with no mode-locked-Q-switched regime, which again improves robustness against damage as in the point before.
A possible disadvantage of the SESAM device according to the invention might be that the device ends on a standing-wave node. Consequently the field strength is maximum at the top surface. This could lead to optical damage of the top surface. A passivating layer on the top surface of the SESAM device could decrease the danger of such optical damage. The passivating layer prevents oxygen and other contaminants from migrating into the semiconductor structure, and also holds in place any contaminants that may already exist on the face of the device. Al the same time, the passivating layer can be made very thin so that it is optically transparent and does not substantially affect the reflectivity and absorption structure of the device. A typical passivating layer for example would consist of a deposition of 2 to 20 nm, preferably 2 to 4 nm, of silicon on the top surface of the SESAM device before it has been removed from its fabrication chamber and exposed to possible contaminants. Passivation techniques for semiconductor laser devices have been disclosed in U.S. Pat. No. 5,144,634, Gasser et al., xe2x80x9cMethod for mirror passivation of semiconductor laser diodexe2x80x9d.